1. Field of the Invention
The present invention relates to a shape measuring apparatus, and particularly, to a shape measuring apparatus for measuring the shape of an object to be measured by simultaneously picking up images of a plurality of phase-shifted interference fringe images obtained by a phase shift method.
2. Prior Art
Conventional measuring apparatuses for measuring the surface shape of an object to be measured include a type that performs a measuring operation in a non-contact manner using the phase shift method. The phase shift method comprises causing a part of light emitted from a light source to be reflected from a reference surface, while causing light passing through the reference surface to be reflected from an observed surface of the measured object, subsequently causing optical images reflected from the reference surface and from the observed surface of the measured object to interfere with each other to induce an interference fringe, then phase-shifting the interference fringes by predetermined amounts to obtain at least three pieces of interference fringe image information from the observed surface of the measured object, and finally capturing these pieces of interference fringe image information by respective corresponding image pickup devices.
As the phase shift method, it is generally employed to vary the phase of interference fringes with the lapse of time using an optical-path varying method of moving the reference surface along an optical axis to physically vary an optical path length by a very small amount or a wavelength varying method of varying a wavelength from a light source by a very small amount with the optical path length fixed.
However, the phase shift method of varying the phase of interference fringes with the lapse of time requires a certain amount of time to obtain at least the interference fringe image information before capturing the interference fringes, on which a subsequent analysis is based. Consequently, not only the amount of time required for the analysis increases but also the results of the analysis are affected by fluctuation, vibration, or the like of air in which the analysis is carried out, thereby making it difficult to keep the results of the analysis reliable.
To solve this problem with the phase shift method of varying the phase of interference fringe images with the lapse of time, a three-way dispersing phase shift method has been proposed, which comprises simultaneously obtaining three interference fringe images of different phases by splitting light from a laser beam source into three beams using two beam splitters, passing these beams through respective corresponding wavelength plates, and then picking up images using three image pickup devices (for example, Japanese Laid-Open Patent Publication (KOKAI) No. 10-281738 and Japanese Laid-Open Patent Publication (KOKAI) No. 11-337321).
However, the three-way dispersing phase shift method enables three interference fringe images to be simultaneously obtained using three image pickup devices, but it is difficult to completely match a sensitivity characteristic or the like between the image pickup devices. The interference fringe images obtained by the image pickup devices actually are different from each other in positional deviation, relative size (bias value) and brightness (contrast value), so that differences in sensitivity characteristic between the image pickup devices are superposed on each other to cause errors, resulting in a failure to obtain accurate shape errors.
Further, since, for example, the conventional three-way dispersing phase shift method requires the use of three image pickup devices, measures must be taken to synchronize these image pickup devices with each other, and moreover, this requires the use of an image input device that can simultaneously capture three pieces of interference fringe image information.
Thus, due to the needs for a plurality of (for example, three) image pickup devices, the above conventional phase shift method has several problems; for example, it is difficult to electrically adjust these image pickup devices and to adjust their physical locations so as to match them with each other, which makes it difficult to achieve accurate shape measurement, and an image pickup system that is necessarily large in size and complicated in structure is required to implement this method.
It is an object of the present invention to provide a shape measuring apparatus which is capable of accurately measuring the shape of an observed surface of an object to be measured and which can be designed to be compact in size and light in weight.
To attain the above object, according to the present invention, there is provided a shape measuring apparatus comprising an interference optical system including a reference surface member having a reference surface, for causing an optical reflected image from an observed surface of an object to be measured and an optical reflected image from the reference surface to interfere with each other to generate interference light, dispersing means for dispersing the interference light generated by means of interference by the interference optical system into at least three beams, shifting means for shifting the at least three dispersed beams by respective predetermined phase shifting amounts, interference fringe image obtaining means for simultaneously obtaining at least three interference fringe images obtained by means of shifting by the shifting means, and calculating means for calculating at least three pieces of interference fringe image information using the at least three obtained interference fringe images and the predetermined phase shift amounts to thereby calculate a shape of the observed surface from the at least three pieces of interference fringe image information, wherein the interference fringe image obtaining means comprises at least one image pickup device which is smaller in number than the interference fringe images.
With the above construction, the interference fringe obtaining means comprises at least one image pickup device which is smaller in number than the at least three interference fringe images, so that the at least three phase-shifted interference fringe images are picked up by the at least one image pickup device smaller in number than the at least three interference fringe images, thereby enabling errors between the individual image pickup devices to be reduced, which errors may occur if the at least three interference fringe images are picked up using at least three image pickup devices. As a result, the size of the observed surface of the measured object can be accurately measured and the entire size and weight of the apparatus can be reduced.
Preferably, the interference fringe image obtaining means comprises a single image pickup device.
Also preferably, the at least three beams have geometrically the same optical path length.
With this construction,- by virtue of the use of the same geometrical optical path length, each of the three phase-shifted interference fringe images can be reliably picked up under the same conditions by the single image pickup device.
Further preferably, refractive-index media having different refractive indices or the like are arranged and disposed on respective corresponding optical paths for the at least three beams such that the optical paths for the at least three beams have optically the same length.
With this construction, by virtue of the use of the same optical path length, each of the three phase-shifted interference fringe images can be reliably picked up under the same conditions by the single image pickup device.